A significant hindrance encountered in thin film production of complex multi-element compositions is the need to employ a multiplicity of alternating deposition and annealing steps for introducing and reacting each of several elements to produce a single layer in a film. Typically, the deposition step is an evaporative or sputtering method, such as any of the physical and chemical evaporative methods known in the art. Such deposition methods are generally costly, and take much energy and time. Moreover, current deposition methods often employ highly toxic chemicals that are difficult to safely handle, such as the use of hydrogen selenide (H2Se) in a selenization step.
For example, in attempting to deposit a thin film of a photovoltaic composition, such as a copper indium gallium diselenide or copper indium gallium disulfide composition (generally, a CIGs composition), conventional practice has been to deposit one or more metals selected from copper, indium, and gallium, followed by selenization with hydrogen selenide. In other conventional methods, an indium-containing ink is deposited on a substrate, reacted with selenium, and then thermally processed to yield a film having a In2Se3 composition. After this step, precursors of copper, indium, and gallium inks are combined to form a mixed copper-indium-gallium ink, which is then deposited on the In2Se3 film before being thermally processed again to provide a CIGs film. As shown, it is not uncommon for current thin film-forming methodologies to take four, five, or more steps, including mixing steps, in depositing such multi-element films.
Furthermore, current thin-film production of complex multi-element compositions is generally not amenable to facile adjustment and optimization of the stoichiometry (i.e., molar ratio) of elements. Nor is current thin-film production of complex multi-element compositions generally known for achieving batch-to-batch repeatability in producing such precise compositions, unless elaborate measures are taken, which are generally not practical for large-scale commercial production.